The tropical Pacific Ocean’s sea surface temperature (SST) remain cooler than average and are in La Niña conditions (Figure A). Most atmospheric indicators of El Niño/La Niña (e.g. trade winds and the Southern Oscillation Index, SOI) are showing patterns suggestive of La Niña conditions. The 1-month Nino3.4 value for December 2017 was -1.0. The 3-month average (October to December) Nino3.4 is at -0.76 which is in the weak La Niña range (Figure B). Partial data in January 2018 show SST anomalies plateauing or warming slightly.

Models suggest the tropical Pacific Ocean will remain cooler than average but weakening (Figure C). Models indicate more than 60% chance of La Niña conditions continuing up to February-April 2018 season (Figure D).

Impact of El Niño/La Niña on Southeast Asia

Typically the impact from La Niña for Southeast Asia is wetter-than-normal rainfall conditions, especially during the Southwest Monsoon period (June – September), including October (Figure E) and especially over the Maritime Continent. During El Niño events the opposite, i.e. drier-than-normal conditions, normally occurs. For November – January season, the impact of El Niño/La Niña is less coherent for some part of the region example over mainland Southeast Asia, Borneo, Southern Sumatra and Malay Peninsula (Figure F). Locally-specific impact differs from place to place and for different seasons.

No two El Niño events or two La Niña events are alike in terms of their impact on the region’s rainfall and temperature. Furthermore, the strength of events and the corresponding impact do not always scale. For example, there were years where relatively weaker El Niño/La Niña events induced more significant changes in rainfall than the stronger events.

Figure A: Sea-surface temperature (SST) anomalies for December 2017 with respect to 1981-2010 climatology. Warm shades show regions of relative warming, while cool shades show regions of relative cooling. The tropical Pacific Ocean Nino3.4 region (solid red box, 120°W-170°W and 5°S-5°N) was cooler than average in December 2017. The western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) was warmer relative to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N), which made the Indian Ocean Dipole Mode index (WTIO minus SETIO) slightly positive but still within neutral levels. Data source: ERSSTv4 from NOAA.

Figure B: The Nino3.4 index using three-month running mean of SST anomalies (against 1981-2010 base period) in the Nino3.4 region bounded by 5°N to 5°S and 170°W to 120°W. Warm anomalies (≥ +0.5) correspond to El Niño conditions while cold anomalies (≤ -0.5) correspond to La Niña conditions; otherwise neutral (> -0.5 and < +0.5). The horizontal axis is labelled with the first letters of the 3-month seasons, e.g. JFM refers to January, February and March seasonal average. Data source: ERSSTv4 from NOAA.

Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (grey) for 2018. La Niña (weak) conditions are favoured over neutral and El Niño up to April 2018, while a return to neutral conditions are expected for the rest of 2018 (image credit: IRI-CPC).

Figure F: November to January rainfall anomaly composite for La Niña years minus El Niño years. Green shades show regions where La Niña induce wetter conditions and El Niño induce drier conditions, while regions in brown shades show the opposite effect, i.e. La Niña inducing drier conditions and El Niño inducing wetter conditions (image credit: IRI Data Library). In general, La Niña events tend to induce wetter conditions for many parts of the region, except for Borneo and Southern Sumatra. Note that this anomaly composite has been generated using limited number El Niño/La Niña occurrences between 1979 and 2016 and therefore should be interpreted with caution.

El Niño/La Niña

For El Niño/La Niña updates, ASMC assesses information provided by the World Meteorological Organization (WMO) and various international climate centres, such as the Climate Prediction Center (CPC) US, the Bureau of Meteorology (BoM) Australia, as well information from the International Research Institute for Climate and Society (IRI) which contains model outputs from various other centres around the world.

Frequently Asked Questions

What is El Niño/La Niña and how do they affect weather in South East Asia?

The El Niño phenomenon is a non-regular occurrence in the tropical pacific region where warmer waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, this brings drier weather and increases the risk of forest fires and smoke haze. The La Niña phenomenon is the reverse of the El Niño where cooler waters develop over the Eastern Tropical Pacific Ocean along the coast of South America.
In South East Asia, higher than normal rainfall tends to occur during a La Niña episode which may result in an increased occurrence of floods.

The image above shows the precipitation anomalies averaged over the El Niño and La Niña years. For instance, the impact of El Niño is typically stronger over the southern and eastern part of South East Asia during the months of Jun – Oct.